Up to now the method used for the preparation of protein G is that described in the above-mentioned publications. It is based on liberating the protein from the surface of streptococcal bacteria with the help of proteolytic enzymes, whereafter the protein is isolated.
This method has serious disadvantages. For example, only a part of the protein G molecule is liberated from the bacterial surface, and even if this part is clearly capable of binding IgG, it would still be advantageous to isolate the whole molecule. Moreover, this method is restricted to the use of streptococci as a starting material, and bearing in mind that these are pathogens and difficult to cultivate on a large scale, it would be desirable to find a method which can apply to other starting materials.
Thanks to the advent of modern genetic engineering, and specifically hybrid DNA technology, improved techniques have become available for the preparation of proteins with particularly attractive properties. The technique begins by locating the genetic information which codes for the desired protein. This genetic information is transferred using vectors from one cell to another. As a result of this transformation, the transformed cell, hitherto unable to synthesize the desired protein, may become capable of producing the desired protein. Such a method has been used for the synthesis of protein A (a cell wall protein of S. aureus) and is described in two Swedish patent applications, Nos. 8204810-9 and 8204811-7, and two European patent applications, Nos. 0 107 509 and 0 124 374.
Compared with protein A, protein G has substantial advantages, especially as a therapeutic agent for removal of antigen-antibody complexes from the blood during extracorporeal blood treatment in connection with certain autoimmune sicknesses. For example, protein G binds to all IgG subclasses, whereas protein A lacks the ability to bind human IgG 3. Furthermore, protein G is a more selective Fc receptor than protein A, since it does not bind immunoglobulin A and immunoglobulin M.
It is therefore an object of the present invention to provide, using hybrid DNA technology, a method for the preparation of protein G and/or fragments of protein G with substantially the same properties as protein G insofar as the capability of binding IgG is concerned. This and other objects have been achieved in accordance with the invention, described in detail below.